1. Field of the Invention
The present invention relates to a method and apparatus for calibrating a laser three-dimensional (3D) digitizing sensor, and in particular to a method and apparatus for calibrating a laser three-dimensional digitizing sensor having the advantages of simple operation, high accuracy and low cost.
2. Description of the Related Art
Optical triangulation systems are widely used to calibrate three-dimensional space. Such calibration systems are applicable in the fields of machine vision and automatic optical inspection, particularly for manufacturing or assembling processes. Conventional calibration of a three-dimensional space is usually expensive due to complicated processes which require high accuracy. Therefore, it is important to provide a precise calibration system with high accuracy, simple operation, and low cost. As shown in FIG. 1, a conventional three-dimensional measurement system comprises a laser emitting device 1, a first optical calibrating set 11, an optical sensor 2, a second optical calibrating set 21 and an object 3. Light emitted from the laser emitting device 1 passes through the first optical calibrating set 11 and is projected onto the object 3 forming a bright point thereon. The optical sensor 2 is similar to a camera generating a digital image by capturing the reflected light passing through the second optical calibrating set 21 from the object 3. Thus, the 3D coordinate of each point on the surface of object 3 can be determined and a complete 3D model can be reconstructed by 3D scanning technology. However, complex parameters and computing processes are inevitably considered and involved during the translation of the digital image data to 3D coordinates. Particularly, when using different optical components to obtain an accurate measurement, it may be difficult to perform the complex processes such as camera parameter estimation, lens distortion compensation, coordinate translation, and laser parameter estimation. Therefore, it is usually expensive and complicated to perform conventional calibration of a three-dimensional space as mentioned above.
To address the disadvantages of conventional 3D calibration, a related art has been disclosed in U.S. Pat. No. 4,925,308 entitled “Calibration of Three Dimensional Space” based on the traditional optical triangulation measurement. The calibration of U.S. Pat. No. 4,925,308 utilizes a calibrating block in a Cartesian X-Y-Z coordinate system, wherein the block has three flat plates respectively in accordance with XY, YZ, and XZ planes perpendicular to each other. To perform the calibration, the optical sensor is preferably sloped at 45 degrees to the flat plates. The calibration the Z axis is accomplished by translating the light projected on the XY plate. Similarly, the X and Y axes can also be calibrated by translating the light projected on the YZ and XZ plates.
The calibration of U.S. Pat. No. 4,925,308 is applied to a point scanning laser sensor only, however, the present invention can provide a calibration method and apparatus for a line scanning laser sensor. In U.S. Pat. No. 4,925,308, reflective conditions and plate flatness dominate the accuracy of measurement, appropriate compensation and modification processes must be employed such that it is more difficult and complicated to calibrate the three-dimensional space. Moreover, the laser sensor is usually sloped by suspension such that it is difficult to install in a large-sized sensor such as a body scanner. The calibration of U.S. Pat. No. 4,925,308 requires a precise positioning system including a high accuracy linear translating platform and flat plates which occupy a large area and are expensive.
To address the disadvantages of the conventional calibration system as mentioned above, the present invention provides a method and apparatus for calibrating a laser three-dimensional (3D) digitizing sensor with simple operation, high accuracy and low cost.